This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-211286, filed Jul. 26, 1999; and No. 11-271117, filed Sep. 24, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a semiconductor infrared detecting device for detecting infrared rays by using the forward characteristic of a pn junction of semiconductor layers. The present invention also relates to an infrared detecting system and an infrared image pick-up system, which employ the device.
A method of measuring temperature by using a pn junction has been realized as a temperature sensor for measuring a low temperature, e.g., within the liquid nitrogen temperature range, but is limited to a point sensor with a relatively large size of several-millimeter-square. In recent years, however, it has become possible to apply the principle of this method to semiconductor infrared image pick-up systems, because micro-machining techniques have been developed and SOI (Silicon On Insulator) substrates with a high quality and a large size have become available.
Semiconductor infrared image pick-up systems utilize a detecting principle in which a thermo-sensing section receives a temperature difference on a target object, and a temperature change in the thermo-sensing section due to this is converted into an electrical signal. Accordingly, the larger the thermo-sensing section is, the higher the sensitivity grows. However, where, for example, thermo-sensing sections each having a size of 50 xcexcm-square and a standard optical system are used while a target object has a temperature difference of 0.1xc2x0 C., only a several-mK change can be caused in each thermo-sensing section. In this case, a signal to be obtained is less than one-hundredth that of conventional point sensors.
In semiconductor infrared image pick-up systems, as the number of arrayed sensors or detecting devices is increased, the diameter of the optical system to be used becomes larger. For example, even if each sensor has a size as small as 50 xcexcm-square, where the sensors are arrayed in the format of 320xc3x97240, the pixel region becomes as large as 16 mmxc3x9712 mm. Due to a trade-off relationship between sensitivity and pixel region size, each pixel size is thought to be 50 xcexcm-square at most. In addition, it is currently demanded to increase the number of pixels, and decrease the size of optical systems, and thus it is thought that each pixel size will be further reduced.
Proc. of SPIE 3698 (1999), pp. 556 to 564, discloses, as a semiconductor infrared detecting device using a pn junction, a structure in which a plurality of pn junctions are formed in one thermo-sensing section and connected to each other in series by using micro-lithography. This structure is conceived to increase a signal voltage to be sufficiently larger than that of noise generated in the thermo-sensing section.
FIG. 15A is a plan view showing the conventional semiconductor infrared detecting device disclosed in the publication, and FIG. 15B is a sectional view taken along line XVBxe2x80x94XVB in FIG. 15A.
This device is formed by using an SOI substrate 210 consisting of a Si substrate 211, an SiO2 film 212, and a p-Si layer (SOI layer) 213. A thermo-sensing section 220 is supported in a floating state by supporting legs 221 and 222 above the Si substrate 211 with a gap 230 interposed therebetween, and thus is thermally independent of the substrate 211. The thermo-sensing section 220 includes a plurality of diodes 215, each of which is constituted by forming an n-diffusion layer 214 in the p-Si layer 213. The diodes 215 are connected to each other by wiring lines 216, and are connected to external sections by wiring lines 218 and 219.
In this structure, a region for converting heat into electrical signals is limited to the pn junctions, and thus, the thermo-sensing section cannot sufficiently increase in temperature, even if a sufficient energy is incident thereon. As a result, it is difficult to form a semiconductor infrared detecting device with a high sensitivity.
Generally, image pick-up devices using a pn junction utilize the reverse characteristic of the pn junction. In this case, dark current should be suppressed at a low value to detect a voltage increase, and thus the surface area of the pn junction does not have to be larger. On the other hand, solar batteries also utilize the reverse characteristic of a pn junction, in which there is a known technique of enlarging the surface area of the pn junction. In this case, the enlarged surface area is adopted to connect small devices to each other in series to increase the output voltage, or to prevent a high-concentration impurity doped layer from being disposed on a surface on which sunlight is incident.
As described above, in conventional semiconductor infrared detecting devices, where a thermo-sensing section becomes larger, the sensitivity is strengthened, but various problems arise. Furthermore, it is currently demanded to increase the number of pixels, and decrease the size of optical systems, and thus the thermo-sensing section tends to be smaller. For these reasons, it is difficult to attain a sufficiently large sensitivity with the conventional semiconductor infrared detecting devices.
An object of the present invention is to provide a semiconductor infrared detecting device with a high sensitivity and a high responsivity, which can improve the sensitivity without making a thermo-sensing section larger.
Another object of the present invention is to provide an infrared detecting system and an infrared image pick-up system, which employ the semiconductor infrared detecting device.
According to a first aspect of the present invention, there is provided a semiconductor infrared detecting device comprising:
a substrate;
a supporting section arranged on the substrate; and
a thermo-sensing section supported by the supporting section above the substrate in a floating state, the thermo-sensing section comprising
a first layer consisting essentially of a semiconductor of a first conductivity type, and
a second layer consisting essentially of a semiconductor of a second conductivity type, and disposed on the first layer to form a pn junction between the first and second layers, the second layer being in contact with the first layer via an interface comprising projections and recesses,
wherein infrared rays are detected with reference to a change in electric current flowing through the pn junction, which is caused when the thermo-sensing section is irradiated with the infrared rays in a state where a bias voltage is applied to the pn junction.
According to a second aspect of the present invention, there is provided a semiconductor infrared detecting system comprising:
the semiconductor infrared detecting device according to the first aspect;
a power supply configured to apply a bias voltage to the pn junction; and
a detector configured to detect electric current flowing through the pn junction.
According to a third aspect of the present invention, there is provided a semiconductor infrared image pick-up system comprising:
a plurality of semiconductor infrared detecting devices according to the first aspect, arrayed in a matrix format;
a power supply configured to apply a bias voltage to the pn junction of each of the detecting devices; and
a detector configured to detect electric current flowing through the pn junction of each of the detecting devices,
wherein an infrared image is picked up with reference to a change in electric current flowing through the pn junctions, which is caused when the thermo-sensing sections are irradiated with infrared rays in a state where a bias voltage is applied to the pn junctions.
According to a fourth aspect of the present invention, there is provided a semiconductor infrared image pick-up system comprising:
a substrate;
a supporting section arranged on the substrate;
a plurality of thermo-sensing sections arrayed in a matrix format, and supported by the supporting section above the substrate in a floating state such that the thermo-sensing sections are thermally substantially independent of each other, each of the thermo-sensing sections comprising
a first layer consisting essentially of a semiconductor of a first conductivity type, and
a second layer consisting essentially of a semiconductor of a second conductivity type, and disposed on the first layer to form a pn junction between the first and second layers, the second layer being in contact with the first layer via an interface comprising projections and recesses;
a power supply configured to apply a bias voltage to the pn junction of each of the thermo-sensing sections; and
a detector configured to detect electric current flowing through the pn junction of each of the thermo-sensing sections,
wherein an infrared image is picked up with reference to a change in electric current flowing through the pn junctions, which is caused when the thermo-sensing sections are irradiated with infrared rays in a state where a bias voltage is applied to the pn junctions.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.